1. Field of the Invention
The invention relates to controls and refers more specifically to structure for and a method of selecting firing points for silicon controlled rectifiers in a contactor of a phase control, current, voltage or power controlling circuit in accordance with variations in line voltage and the circuit power factor as determined by circuit resistance and impedance, specifically inductive impedance.
2. Description of the Prior Art
Electronic controls have been utilized for example in resistance welders to provide phase control firing of thyristors, either silicon controlled rectifiers or ignitron tubes, in contactor structure in the primary circuit of a welding transformer to control the amount of welding current in the secondary circuit. The purpose of controlling the firing point of the thyristors of the contactor has been to control the current to a weld to provide a known and stable energy flow for heating the metal to a desired temperature. It is desirable to apply energy to a weld at a rate neither too high nor too low, in order to secure a good weld. It is further desirable to keep the energy rate from changing due to external influences, such as power line voltage fluctuation and power factor fluctuation of the welding circuit itself.
With prior resistance welding circuit control structure, it has not been possible to adequately correct for line voltage fluctuations and power factor fluctuations so that undesirable welds were often produced, resulting in an undesirable percentage of industrial scrap.